The grid’s increasing dependence on intermittent solar and wind resources is a main factor in the expansion of energy storage systems, particularly large-scale installations. Lithium-ion batteries became the dominant solution due to rapidly declining costs and high energy density. However, unlike aqueous electrolyte batteries, lithium-ion chemistries are inherently susceptible to thermal runaway, an uncontrollable self-heating state that produces extreme temperatures and flammable gasses (like hydrogen). One defective cell may cause cell-to-cell thermal propagation. Several major energy storage installations experienced catastrophic fires in the past decade, causing first responder injuries and environmental contamination. Likewise, failure of residential systems can also result in loss of life and/or property.
The losses of life and property resulting from fires associated with lithium-ion batteries have resulted in a flurry of activity to create safer products and more stringent requirements for installation and operation. In particular, changes that have been made across two key standards: NFPA 855 and UL 9540.
The Interplay of ESS Codes and Standards
UL 9540, Standard for Energy Storage Systems and Equipment, is the system-level safety standard for energy storage systems (ESS) – a certification required by both the National Electrical Code (NEC) and NFPA 855, Standard for the Installation of Stationary Energy Storage Systems. The first edition of UL 9540 was published in 2016. Before its release, manufacturers and inspectors had to rely on component-specific safety standards, as there was no system-level certification for ESS. Batteries are evaluated under UL 1973, and inverters and power conversion equipment are covered by UL 1741; UL 9540 created an overarching framework by referencing these component-level standards, adding additional requirements, and providing, for the first time, a comprehensive ESS certification rather than just individual component listings.
NFPA 855 serves as the primary standard for managing the risks associated with ESS. While not all ESS are electrochemical, NFPA 855 focuses heavily on the hazards of chemical energy storage and lithium-ion batteries. Its scope states it “applies to the design, construction, installation, commissioning, operation, maintenance, and decommissioning of stationary energy storage systems (ESSs), including mobile and portable ESSs installed in a stationary situation and the storage of lithium metal or lithium-ion batteries.”
In regards to ESS, the National Electrical Code leans heavily on UL 9540 and NFPA 855 to provide installation requirements. Fortunately, the most recent editions of the NEC, UL 9540, and NFPA 855 have seen a great deal of language harmonization, making deployments more straightforward for installers and the enforcement community alike.
UL 9540 Standard for Energy Storage Systems and Equipment
The development of UL 9540 was driven by lithium-ion batteries with a higher hazard potential than aqueous electrolyte batteries (such as lead-acid and nickel metal hydride) rapidly entering the commercial and residential markets. Certifying individual components wasn’t perceived as stringent enough to prevent potentially catastrophic failures. A standard evaluating how the battery, power conversion system, battery management system (BMS), thermal management, and enclosures all interacted as a unified product was needed to give Authorities Having Jurisdiction (AHJs) a straightforward way to confidently verify safety and permit installations.
As the energy storage market grew over the last decade, including the advent of massive, grid-scale systems, UL 9540 underwent several revisions to address hazards unique to large installations and provide more installation-specific details for AHJs.
First Edition (2016): The inaugural edition established the system-level safety framework. It laid out mechanical, electrical, and environmental tests, but it was drafted in an era where ESS were generally smaller in size.
Second Edition (2020): By the time the second edition was published, utility-scale, containerized ESS had become common, bringing new fire hazards with them. This edition shifted focus to ensure fire propagation behavior was treated as a core safety attribute rather than an afterthought. It integrated the requirements for UL 9540A, a destructive test method used to gather data on how cells and battery units behave when thermal runaway propagates (see below).
Third Edition (2023): The most recent revision of the third edition added Annex H, a template for requirements that manufacturers must provide as installation instructions for residential ESS. This should prove extremely helpful for both installers and AHJs going forward as it addresses NEC and NFPA 855 requirements directly and succinctly, including minimum separation distances from other units, doors, and windows.
UL 9540A Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems
The UL 9540A test method was first published in 2017 and is currently on its 6th edition (2026). This testing is required for a majority of installation conditions by both the UL 9540 standard itself, and by NFPA 855. It contains test methods for cells, modules, and both residential and non-residential lithium-ion ESS units. It is required for units with capacities >50kWh; or with allowable separation distances less than 3 feet to adjacent ESS units, doors, windows, or exposures; or for units allowed to be located indoors on or in dwelling units.
A majority of lithium-ion ESS manufacturers pursue UL 9540A testing even if it is not strictly required for all use cases (for example a single, outdoor-only 40kWh commercial unit). It adds flexibility for integrators, particularly in regards to the minimum 3-foot unit spacing separation that would otherwise be required.
The 6th edition revision also incorporates installation-level “large-scale fire testing” for units >20kWh. This is an ignition-of-gases test and is harmonized with 2026 NFPA 855 (more details below). It also includes Annex C, a large-scale deflagration test for BESS enclosures required if the enclosure does not have deflagration pressure relief area(s) designed in accordance with NFPA 68, Standard on Explosion Protection by Deflagration Venting.
NFPA 855 Standard for the Installation of Stationary Energy Storage Systems
The development of NFPA 855 began in 2016 in response to increasing deployments of ESS technologies and a recognition that the rapid evolution of energy storage technology was outpacing existing codes.It was initially based on 2018 and 2021 International Fire Code (IFC) requirements. The idea was to unify safety requirements, mandating specific guidelines for what capacity and types of ESS can be located where; what fire suppression and detection systems must be in place; and what commissioning, operations, and maintenance procedures are required. After receiving over 600 public inputs and 800 comments during its development, the first edition of 855 was published in 2020. The 2027 IFC will refer to NFPA 855 as the primary requirements for ESS installations.
First Edition (2020): The inaugural version laid the foundational safety protocols for installation of ESS,emphasizing the requirement for UL 9540 listing. It set baseline rules for lithium-ion installations including thermal runaway and explosion control; maximum energy capacities per unit and aggregate quantities; separation distances; requirements for fire detection and suppression; and commissioning, maintenance, and testing.
Second Edition (2023): The second edition expanded the scope and refined previous rules based on public comments and new field data. This edition provided better definitions for various battery systems; adjusted separation distances based on large-scale fire testing; and clarified the application of explosion control measures (such as deflagration venting) to prevent the dangerous build-up of explosive gases.
Third Edition (2026): The 2026 edition removed seemingly arbitrary aggregate capacity limits for non-residential electrochemical ESS in Chapter 9, and instead relies on broader requirements for fire and explosion testing, hazard mitigation analysis, and emergency operations plans. For residential ESS (covered in Chapter 15), aggregate capacity limits were increased to 100kWh in attached garages, 200 kWh on exterior walls or on the ground, and to 600kWh when outdoors and ≥10 feet from the dwelling and property lines.
If flammable gases are released during UL 9540A cell-level testing (as is expected with lithium-ion cells), 2026 NFPA 855 now requires an additional unit-level test with intentional ignition of the vent gases to show that a fire involving one ESS unit will not propagate to an adjacent unit. As mentioned above, the 6th Edition of UL 9540A includes this unit-level ignition test as described in 855.
What’s next?
Just like technological development, standard development never stops.
The 2029 edition of NFPA 855 is already under development, and the public input closing date (June 3, 2026) has already passed. Key dates that are up next include:
- First draft posting date: March 24, 2027
- Public comments closing date: June 2nd, 2027
- Second draft posting date: March 1, 2028
UL 9540 currently has task groups working on two issues the standard is anticipated to address in the future:
- Residential (<20 kWh unit) large-scale fire (installation-level) testing that will likely become an alternative to unit-level tests
- 2nd life/repurposed batteries (primarily EV batteries)
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